What's the Purpose of a Cam?

It make the rockers go wiggly waggly and close and open the valves. Mostly just in time.

specially for "in time" ..

Here's an animation of an internal combustion engine. Note the interaction between the pistons, cams and valves about halfway through the movie.

See also here and here.

In general the function of cam is to convert rotery motion in to linear (reciprocating ) motion..

http://www.flying-pig.co.uk/mechanisms/pages/cam.html go though this link for more understanding..